1. Field of the Invention
This invention relates to a surface plasmon sensor for quantitatively analyzing a material in a sample utilizing generation of surface plasmon, and more particularly to an Otto type surface plasmon sensor.
2. Description of the Related Art
In metal, free electrons vibrate in a group to generate compression waves called plasma waves. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon sensors for quantitatively analyzing a material in a sample utilizing a phenomenon that such surface plasmon is excited by light waves. Among those, one employing a system called "Kretschmann configuration" is best known. See, for instance, Japanese Unexamined Patent Publication No. 6(1994)-167443.
Further there has been known also a surface plasmon sensor generally referred to as "an Otto type surface plasmon sensor". The Otto type plasmon sensor basically comprises a dielectric block shaped, for instance, like a prism, a metal film having a sample supporting side which is faced toward one face of the dielectric block spaced therefrom and on which a sample is placed, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block so that the light beam is reflected in total reflection at said one face of the dielectric block and various angles of incidence of the light beam to said one face of the dielectric block including an angle of incidence at which surface plasmon is generated can be obtained, and a detecting means which detects the angle of incidence of the light beam at which attenuation in total reflection takes place and the amount of reflected light is reduced.
In order to obtain various angles of incidence of the light beam to the interface, a relatively thin light beam may be caused to impinge upon said one face of the dielectric block while deflecting the light beam or a relatively thick light beam may be caused to converge on said one face of the dielectric block so that components of the light beam impinge upon the interface at various angles. In the former case, the light beam which is reflected from the face at an angle which varies as the light beam is deflected may be detected by a photodetector which is moved in synchronization with deflection of the light beam or by an area sensor extending in the direction in which reflected light beam is moved as a result of deflection. In the latter case, an area sensor which. extends in directions so that all the components of light reflected from the interface at various angles can be detected by the area sensor may be used.
In such an Otto type plasmon sensor, when a sample is fixed on the sample support side of the metal film having a sufficient thickness and a light beam is caused to impinge upon said one face of the dielectric block opposed to the sample support side of the metal film at a particular angle of incidence .theta.sp not smaller than the angle of total internal reflection, evanescent waves having an electric field distribution are generated in the sample and the metal film, and surface plasmon is excited in the metal film. When the wave vector of the evanescent waves is equal to the wave number of the surface plasmon and wave number matching is established, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light reflected in total reflection from the interface of the dielectric block and the metal film sharply drops.
When the wave number of the surface plasmon can be known from the angle of incidence .theta.sp at which the phenomenon of attenuation in total reflection takes place, the dielectric constant of the sample can be obtained. When the dielectric constant of the sample is known, the concentration of a specific material in the sample can be determined on the basis of a predetermined calibration curve or the like. Accordingly, a specific component in the sample can be quantitatively analyzed by detecting the angle of incidence .theta.sp at which the intensity of light reflected in total reflection from the face of the dielectric block sharply drops.
Though being advantageous in that measurement is easy, Kretschmann type surface plasmon sensor is disadvantageous in that it is necessary to precisely control the thickness of the metal film and the photo-coupler for causing total reflection must be in refractive index matching with the sensor.
To the contrast, in the Otto type surface plasmon sensor, the aforesaid resonance is caused constantly under the same condition so long as the thickness of the metal film is sufficient and measurement can be effected without contact with the sample.
However, in order to effect accurate measurement by use of the Otto type surface plasmon sensor, it is necessary to keep constant the distance between said one face of the dielectric block and the sample support side of the metal film within the range by which the evanescent waves ooze from the face of the dieletric block, which makes the measurement very difficult.